The invention relates to a curtain coating device for fiber machines, in particular for paper and board machines. More especially the invention relates to a curtain coating device having a nozzle beam with a nozzle lip with an edge strip for feeding the coating color curtain and edge guides located along each edge of the nozzle lip to control the width of the coating color curtain, wherein each edge guide comprises upper and lower ends, flow surfaces extending in a substantially vertical direction parallel and next to each other, a lubrication feed, additional lubrication feeds and a suction opening.
Coated fiber web grades and coating are becoming more and more popular and thus the coating process and equipment have increasing demands imposed thereon. In coating, especially in pigment coating, the surface of a fiber web is formed with a layer of coating color (coating agent) at a coating station followed by drying. The process of coating can be divided in supplying the coating color onto the web surface, which is called the application of the coating color, as well as in the adjustment of final amount of coating color. One important recently developed coating technique is curtain coating, which is suitable to coat paper and board. By curtain coating good coverage of coating color on the web surface to be coated is achieved.
In curtain coating two main types of curtain coating devices are used, namely slot-fed curtain coating devices and slide-fed curtain coating devices. In the slide fed curtain coating devices, coating color is fed by means of a nozzle assembly onto an inclined plane and the coating color flows down toward an edge of the plane constituting a nozzle lip and the curtain is formed as the coating color falls off the nozzle lip onto the web. In the slot-fed curtain coating devices coating color is pumped through a distribution chamber into a narrow vertical slot and the curtain is formed at its lip and falls onto the web. Coating can be applied in one or more curtain layers.
Typically two different methods are used to control width of the curtain. One is to feed the curtain wider than the width of the fiber web to be coated and then the width is allowed to freely narrow in the cross direction of the web as the curtain falls downwards toward the fiber web. This method is called the out-board method. The other, called the in-board method, is to feed the curtain narrower than the width of the fiber web to be coated, and the width is required to be maintained uniform as the curtain falls downwards toward the fiber web. The curtain is maintained at the required width by means of edge guides which are located along each edge of the feeding slot/the nozzle lip. An edge guide typically comprises a flow surface for guiding the edge of the coating curtain, a lubrication liquid (typically water) supply(s) onto the edge guide, a collecting lip and a suction channel for collecting and removing edge area coating color and lubrication liquid. The present invention relates to in-board curtain coating and to the edge guides used to keep the curtain at a required width.
WO publication 03/049870 discloses an edge guide for a curtain coater which has at least one contact area of its surface directed toward a coating curtain, which contact area has a multiplicity of grooves and ribs extending along the length of the edge guide.
EP patent publication 1817115 discloses an edge guide for a curtain coating device having a guide surface with a guide groove, at least one liquid feed and a quietening section.
EP patent publication 1900441 discloses an edge guide of a curtain coating device, comprising liquid supply means, wherein a member constituting a surface of the edge guide, which faces the coating color layer is a porous material member.
U.S. Pat. No. 7,160,579 discloses an edge guide for a curtain coating device with a groove having incorporated in its surface channels parallel to the direction of the falling curtain and the curtain is stabilized with liquid flow which is supplied to the groove.
One problem in all known edge guides for curtain coating devices is stability of the curtain on the edge guide, especially when the feed amounts of the coating color are low.
Another problem is adhering and drying of the coating color onto the edge guide and especially onto the suction channel of the edge guide, which disturbs the coating process.
Problems at edge areas of the curtain are mainly related to four different factors to which construction of the edge guide effects: first, coating color thickness on the area between the sliding surface of the nozzle beam and the edge strip; second, encounter location of three boundary surfaces where the edge guide is connected to the nozzle beam; third, material, form, and inclination angle of the edge guide and lubrication/moisturizing liquid fed onto the edge guide; and fourth, removal of lubrication/flushing liquid by suction at the lower edge of the edge guide. The present invention relates to the latter three of the above mentioned factors.
In known edge guides for curtain coating devices the encounter location of three boundary surfaces where the edge guide is connected to the nozzle beam is problematic, since this encounter location of three boundary surfaces causes disturbances to the flow of the curtain and differences in surface energy of the surfaces cause problems. The three surfaces include the nozzle beam (the lip of the nozzle), the edge guide and the edge strip.